Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a specimen. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. When inspecting specular or quasi-specular surfaces such as semiconductor wafers bright field (BF) and dark field (DF) modalities may be used, both to perform patterned wafer inspection and defect review. In BF inspection systems, collection optics are positioned such that the collection optics capture a substantial portion of the light specularly reflected by the surface under inspection. In DF inspection systems, the collection optics are positioned out of the path of the specularly reflected light such that the collection optics capture light scattered by objects on the surface being inspected such as microcircuit patterns or contaminants on the surfaces of wafers. Viable inspection systems, particularly BF inspection systems, require high radiance illumination and a high numerical aperture (NA) to maximize the defect sensitivity of the system.
Current wafer inspection systems typically employ illumination sources of deep ultraviolet (DUV) radiation with a high numerical aperture (NA). In general, the defect sensitivity of an inspection system is proportional to the wavelength of the illumination light divided by the NA of the objective. Without further improvement in NA, the overall defect sensitivity of current inspection tools is limited by the wavelength of the illumination source.
In some examples of BF inspection systems, illumination light may provided by an arc lamp. For example, electrode-based, relatively high intensity discharge arc lamps are used in inspection systems. However, these light sources have a number of disadvantages. For example, electrode based, relatively high intensity discharge arc lamps have radiance limits and power limits due to electrostatic constraints on current density from the electrodes, the limited emissivity of gases as black body emitters, the relatively rapid erosion of electrodes made from refractory materials due to the presence of relatively large current densities at the cathodes, and the inability to control dopants (which can lower the operating temperature of the refractory cathodes) for relatively long periods of time at the required emission current.
To avoid the limitations of electrode based illumination sources, incoherent light sources pumped by a laser (e.g., laser sustained plasma) have been developed. Exemplary laser sustained plasma systems are described in U.S. Pat. No. 7,705,331 assigned to KLA-Tencor Corp., which is incorporated by reference as if fully set forth herein. Laser sustained plasmas are produced in high pressure bulbs surrounded by a working gas at lower temperature than the laser plasma. Substantial radiance improvements are obtained with laser sustained plasmas. Atomic and ionic emission in these plasmas generates wavelengths in all spectral regions, including shorter than 200 nm when using either continuous wavelength or pulsed pump sources. Excimer emission can also be arranged in laser sustained plasmas for wavelength emission at 171 nm (e.g., xenon excimer emission). Hence, a simple gas mixture in a high pressure bulb is able to sustain wavelength coverage at deep ultraviolet (DUV) wavelengths with sufficient radiance and average power to support high throughput, high resolution BF wafer inspection.
Development of laser sustained plasmas has been hampered by issues related to plasma instability caused by unpredictable working gas flows within the bulb. In particular, turbulent gas flows create uncertainty in the position of plasma and distort the plasma profile itself. In addition, the unpredictable gas flows destabilize the optical transmission properties of the working gas. This creates additional uncertainty in the pump light that reaches the plasma and the light extracted from the plasma.
Traditional laser sustained plasma light sources rely on natural convection to cool the laser sustained plasma. Structures may be added to influence the natural convective flows. However, the flow rates through the plasma are typically limited to approximately one meter per second, and flow instability and uncertainty introduces significant noise into the illumination system. Pressurized gas flow can be incorporated to increase the flow rate of working gas through the plasma, but control over the flow field within the plasma bulb is limited to regions near the exit of the nozzle delivering the pressurized flow. Typically, the nozzle must be located relatively far from the plasma. This limits precise control of the flow field in the plasma by pressurized gas flow itself. In addition, the mechanical systems employed to generate the pressurized working gas tend to have a relatively slow response time that limits their ability to be used as part of an active, real time flow control scheme to stabilize the laser sustained plasma.
As inspection systems with laser sustained plasma illumination sources are developed, maintaining plasma stability becomes a limiting factor in system performance. Thus, improved methods and systems for controlling working gas flows within a laser sustained plasma light source are desired.